1. Field of the Invention
The present invention relates to the manufacture of a solid state electrochemical cell, and more particularly, a rechargeable solid state laminar cell including a lithium anode which is capable of generating a high current density.
2. Description of the Prior Art
Solid state electrochemical rechargeable chemical cells are the subject of intense investigation and development. They are described extensively in the patent literature. See, for example, U.S. Pat. Nos. 4,303,748 to Armand; 4,589,197 to North; 4,547,440 to Hooper, et al; and 4,228,226 to Christian. These cells are typically constructed of an alkali metal foil anode, typically lithium foil, an ionically conducting polymeric electrolyte, a composite cathode containing a finely divided transition metal oxide and an electrically-conductive filler, and a current collector. These cells present a viable option to older, more traditional, secondary cells because of their ability to generate a relatively high current per unit area and high storage capacity.
To date, solid state lithium anode batteries which have been described in the literature have only been capable of discharging current at a rate of between about 0.05 ma/cm.sup.2 and 10 ma/cm.sup.2. Efforts are continuing to be made to improve the efficiency of these cells.
According to Ohm's law, the voltage drop of a cell is equal to the current multiplied by the cell's internal resistance (impedance). Accordingly, for a solid state lithium cell having a voltage of 2.7 volts, if the overall impedance of the cell is reduced, the amount of current available from the cell is correspondingly increased, making it a more attractive commercial alternative.
In solid state alkali metal anode laminar cells, internal cell impedance is the product of a number of factors. An internal impedance is associated with each element of the cell, namely the anode layer, the electrolyte layer, the cathode layer, and current collector. Further, and is particularly problematic, high impedance can occur at the interfaces between these layers and, more specifically, at the anode/electrolyte interface, at the electrolyte/cathode interface and at the cathode/current collector interface. Accordingly, to produce an efficient solid state electrochemical laminar cell which is capable of delivering high current density and storage capacity, the impedance of each of these layers and interfaces must be minimized. The present invention seeks to provide as small an internal impedance as possible.
In addition to providing a cell having a minimum internal impedance, it is also desirable to produce a cell capable of operating across a wide temperature range. For example, presently available solid state lithium anode laminar cells operate very inefficiently at or below room temperature.
Accordingly, there exists a need in the art for a solid state alkali metal anode laminar cell which has a low overall impedance, and is capable of discharging a high amount of current per unit area over a broad temperature range.